Epimastigotes of T. cruzi are lysed by the alternative complement pathway in normal human serum, whereas metacyclic trypomastigotes (CMT) and tissue culture trypomastigotes (TCT) resist lysis. 5-8 fold more C3 and C9 bind to Epi than to CMT or TCT duration incubation in NHS. We have shown that the major molecule to which C3 binds in Epi is a 72 Kd developmentally regulated glycoprotein, GP72, whereas the C3 acceptors in CMT are 40 Kd and 225 Kd molecules. The major form of C3 on Epi is C3b whereas the major form on CMT and TCT is hemolytically inactive iC3b. Inefficient alternative pathway activation by CMT is due to poor factor B binding to C3b on the parasite. Pronase rendered CMT sensitive to ACP lysis and resulted in enhanced B and C3 binding. CMT labeled by periodate oxidation and 3H Na BH4 reduction showed a 90 Kd glycoprotein not labeled in Epi, a molecule which is specifically removed by pronase. Intrinsic labeling of CMT and TCT with 35S methionine and subsequent incubation in buffer showed that this 90 Kd band was shed in almost pure form into the supernatant. Both CMT and TCT supernatant, as well as parasite lysates, contained components which were capable of inhibiting formation and accelerating decay of the alternative pathway C3 convertase. We are currently characterizing the molecules responsible for this decay accelerating activity. C3 binding was analyzed on promastigotes of L. donovani. The predominant form of C3 was iC3b. C3 bound to a parasite derived acid phosphatase by hydroxylamire resistant (putative amide bond) but was rapidly released from the surface of the parasite by an unusual cleavage event. Similar release of C3 was found with non-infective, serum sensitive, log phase promastigotes of L. major but minimal release was detected from infective, serum resistant stationary phase L. major. Preliminary results show C3 binding to human erythrocytes infected with Plasmodium falcipacium.